Field of the Invention
This invention relates to the isomerization of hydrocarbons and more particularly to the vapor phase isomerization of hydrocarbons by means of a metal-containing catalyst which is periodically regenerated to remove carbonaceous deposits.
The term "isomerization" is used herein to describe the conversion of a hydrocarbon into at least one different hydrocarbon product having the same number of carbon atoms. Isomerization is particularly useful for the conversion of straight chain paraffinic hydrocarbons such as n-butane into isobutane and for the conversion of aromatic hydrocarbons such as metaxylene into a mixture of orthoxylene and paraxylene.
Hydrocarbon isomerization is carried out commercially by contacting the hydrocarbon to be isomerized with a catalytic material at elevated temperatures and at pressures ranging from atmospheric to several hundred psi. The catalyst usually has a relatively long life since it is not, to any great extent, permanently affected by the isomerization reaction. However, when isomerization is carried out in the vapor phase there is a rapid buildup of carbonaceous deposits, such as coke, on the catalyst. These deposits must be frequently removed as they rapidly inactivate the catalyst. The catalyst is regenerated by burning off the carbonaceous deposit in the presence of free oxygen. During the catalyst regeneration carbon monoxide and carbon dioxide are produced as gaseous combustion products, the relative amounts of each depending upon the efficiency of the oxidation step.
Recent federal and local air pollution control legislation has made it necessary to eliminate or drastically reduce the amount of carbon monoxide being discharged into the atmosphere. One method of accomplishing this is by contacting the gaseous effluent from the catalyst regenerator with an oxidation promoter to convert the carbon monoxide to carbon dioxide, however the cost of this operation is too great for economically practical operation of all but very large plants in which the production of carbon monoxide is great enough to justify the additional expenditure for a carbon monoxide conversion unit.
The elimination of carbon monoxide as a gaseous by-product from hydrocarbon conversion processes using a regenerative catalyst can also be effected by using a catalyst which contains a promoter, usually a metal or metal compound, for the conversion of carbon monoxide to carbon dioxide. U.S. Pat. No. 2,647,860 describes a hydrocarbon cracking process using a synthetic silicaalumina catalyst which is capable of regeneration, and and which contains about 0.01 to 1% by weight of chromic oxide to promote the oxidation of carbon monoxide in the catalyst regenerator. This patent acknowledges the fact that the chromic acid adversely affects the cracking reaction. U.S. Pat. No. 3,140,249 describes the preparation and use of crystalline aluminosilicate compositions containing small amounts of metals such as vanadium and manganese in hydrocarbon cracking processes. U.S. Pat. No. 3,140,253 discloses the preparation and use of crystalline aluminosilicates in hydrocarbon conversion processes, including hydrocarbon isomerization. This patent states that metal promoters such as chromium, cobalt and platinum may be present in the catalyst in amounts of about 5 to 40% by weight. U.S. Pat. No. 3,173,854 describes a process for the hydrocracking of hydrocarbon oils using a crystalline aluminosilicate containing, as a hydrogenation component, 0.01 to 25% by weight of a metal selected from Groups VI and VIII of Periodic Table. U.S. Pat. No. 3,650,990 discloses the preparation of a crystalline aluminosilicate useful for the conversion of hydrocarbons containing a metal promoter selected from Groups IVB, VB, VIB, VIIB and VIII of the Periodic Chart. The metal is incorporated into the crystalline aluminosilicate prior to or during its formation and may vary from .001 to 5% by weight. U.S. Pat. No. 3,788,977 describes hydrocarbon cracking using a platinum-impregnated crystalline aluminosilicate catalyst. The platinum which is present in amounts of 0.5 ppm or more increases the yield of aromatic product. The use of catalyts which contain carbon monoxide oxidizing metals or metal compounds in isomerization processes significantly reduces the efficiency of the catalyst for the hydrocarbon isomerization reaction.
U.S. Pat. No. 3,364,136 describes hydrocarbon conversion using a catalyst comprised of a first component consisting of solid particles containing a crystalline aluminosilicate which is active for the conversion of the hydrocarbon, and a second component consisting of solid particles containing a crystalline aluminosilicate which is inert with respect to the hydrocarbon conversion reaction because its pores are too small to admit the hydrocarbon reactant, but which is effective to promote oxidation of carbon monoxide to carbon dioxide. The latter component contains from less than 0.01 up to 20% by weight of a metal, metal oxide or metal sulfide from Groups IB, IIB, VIB, VIIB or VIII of the Periodic Chart. The disadvantage of this catalyst is that its use reduces the capacity of the reactor since only a portion of the catalyst is useful for hydrocarbon conversion. Belgian Pat. 820,181 describes a process for cracking hydrocarbons with a cracking catalyst material which contains less than the 100 ppm of a metal chosen from Periods 5 and 6 of Group VIII of the Periodic Chart or rhenium. This patent teaches that the metal may be applied uniformly over all of the catalyst or it may be applied at a higher concentration on a portion of the catalyst and blended with unmodified catalyst provided that the total metal content of the final catalyst mixture does not exceed 100 ppm.